MODELING AND SIMULATION OF COAL GASIFICATION 11 IN CIRCULATING FLUIDIZED BED REACTOR

Abstract

The present work aims to develop a CFD model of circulating fluidized bed reactor (CFBR) for coal gasification to study the hydrodynamics phenomenon inside the gasifier. For this purpose, a two dimensional model of the CFBR having a diameter of 3.5 m and height of 17.6 m has been developed and quadrilateral cells are selected for discretization of the geometry. Simulation of meshed geometry of has been performed within the frame work of ANSYS 13.0 software Eulerian—Eulerian modeling approach is used to describe mass, momentum and energy transfer between gas and solid phase. Syamlal- O'Brien drag model is used to study the hydrodynamics inside the gasifier. Phase couple scheme with Green-Gauss cell based method is used as a spatial discretization approach. The mechanism of solid phase has been described by the kinetic theory of granular flow (KTGF) approach. This gasification model includes the mechanism of drying, volatilization, gasification and combustion processes inside the gasifier. Gasifier involves the kinetics of four heterogeneous reactions (char combustion reaction, char carbon dioxide combustion reaction, char hydration reaction, char hydrogenation reaction) and five homogeneous reactions (methane combustion reaction, carbon monoxide combustion reaction, hydrogen combustion reaction, water gas shift reaction, reverse water gas shift reaction). Eddy dissipation model of FLUENT describes the kinetics of homogeneous reactions and a user defined code written in C, which involves the Arrhenius kinetics; describe the kinetics of heterogeneous reactions. Calibration and validation of the model has been done using literature. Results shows the hydrogen concentration as well as carbon monoxide concentration decreases along the height of gasifier whereas the concentration of carbon dioxide increases with increase in reactor height, possibly due to the combustion of carbon monoxide to carbon dioxide inside the gasifier. The temperature inside the gasifier also decreases with reactor height due to decrease in the rate of exothermic reactions inside the gasifier. The model has also been used to study the effect of temperature on the various species of syngas as well as to see the fluidization phenomenon inside the gasifier. For the investigation of the effect of temperature inside the gasifier, kinetic modeling has been done within the framework of MA1'LAB software. Carbon monoxide and hydrogen concentration increases with increase in temperature whereas the concentration of carbon dioxide and water vapor decreases with increase in temperature.